1. Field of the Invention
The present invention generally relates to an induction heating device that heats a heated body (i.e., a heating target member) by induction heating, an induction heating fixing device including the induction heating device, and an image forming apparatus including the induction heating device.
2. Description of the Related Art
In an image forming apparatus such as a copier and a printing device employing an electrophotographic process, an image is formed by transferring a toner image onto a sheet and then heating the sheet by a fixing roller as a fixing means, the toner image having been formed on a photosensitive body.
Recently, it has become more and more important to address environmental concerns, and accordingly energy conservation in an image forming apparatus has been improving. To improve the energy conservation in the image forming apparatus, it may be necessary to reduce the energy consumption in the fixing device that melts and adheres toner to the sheet.
In response to the demand for reducing the energy consumption, recently, there has been employed an induction heating fixing device 108 as illustrated in FIG. 1, which includes an exciting coil 101, a heating roller 102, a fixing/pressing roller 103, an induction heating driver circuit 104 to control a drive current to the exciting coil 101, and a temperature sensor 105 to detect a temperature of the heating roller 102. Among those elements, it is known that the exciting coil 101, the induction heating driver circuit 104, and the temperature sensor 105 constitute the induction heating device.
The induction heating fixing device 108 heats the heating roller 102 by generating eddy currents in a heat generation layer (electrical conducting layer) of the heating roller 102 by using magnetic flux generated by the exciting coil 101, and transfers the heat of the heating roller 102 to the fixing/pressing roller 103. In the meantime, by feeding a sheet 107 between the heating roller 102 and the fixing/pressing roller 103, the toner 106 mounted on the sheet 107 is melted and adhered to the sheet 107. In this case, the temperature of the heating roller 102 is detected by the temperature sensor 105 provided near the heating roller 102, so that the induction heating driver circuit 104 controls the temperature of the heating roller 102 at a predetermined (desired) temperature.
Recently, the induction heating fixing device 108 having the configuration described above has attracted attention because of having remarkably shorter time period necessary to increase the temperature to the operating temperature and also having higher efficiency so as to contribute to reducing environmental impacts.
FIG. 2 is a schematic circuit diagram including the induction heating driver circuit 104.
As illustrated in FIG. 2, an AC (Alternating-Current) Voltage input from a commercial power source 201 passes through a noise filter circuit 202 including capacitors C1, C2, and C3 and a common-mode choke coil L1 and is full-wave rectified by a diode bridge DB1.
The full-wave rectified AC voltage is converted (smoothed) into a direct current (DC) by an LC filter circuit 203 including capacitors C4 and C5 and a choke coil L2 and is input to one end of a resonance capacitor Cres. The other end of the resonance capacitor Cres is connected to the collector of a switching device Q1 made of an IGBT (Insulated Gate Bipolar Transistor) or the like. In this case, the emitter of the switching device Q1 is connected to ground (GND).
The ends of the resonance capacitor Cres are connected to corresponding ends of the exciting coil 101 via two wires and an external connector CN1, so that the exciting coil 101 and the resonance capacitor Cres constitute an LC parallel resonance circuit.
A drive circuit 206 of a control circuit 204 outputs a drive signal to the base of the switching device Q1. By turning on and off the switching device Q1 by the drive signal from the control circuit 204, a high-frequency current flows to the exciting coil 101. As a result, the magnetic flux is applied to the heating roller 102 and the eddy currents are generated in the surface of the heating roller 102 to generate heat in the heating roller 102.
As illustrated in FIG. 2, the control circuit 204 includes an input power detecting section 205, a control section 207, and the drive circuit 206. The input power detecting section 205 detects input AC power based on detection signals from an input current detecting circuit 205a and an input voltage detecting circuit 205b. The control section 207 calculates an appropriate pulse width (length) based on the output from the input power detecting section 205 and a temperature detecting signal from the temperature sensor 105. The drive circuit 206 drives the switching device Q1 based on a signal from the control section 207.
The LC parallel resonance circuit including the exciting coil 101 and the resonance capacitor Cres, the switching device Q1, and a diode D1 of the switching device Q1 constitute a voltage resonance (type) inverter. The operations of the voltage resonance (type) inverter are described with reference to FIGS. 3 and 4.
FIG. 3 schematically illustrates transitions of the on/off states of the switching device Q1 and relationships between the on/off states and the corresponding currents flowing through any of the exciting coil 101, the resonance capacitor Cres, the switching device Q1, and the diode D1. On the other hand, FIG. 4 schematically illustrates waveforms of the drive voltage VG of the switching device Q1, a voltage between the collector and the emitter Vce, and a high-frequency current IL flowing through the exciting coil 101. In FIG. 3, the parts drawn using the dashed dotted lines indicate the parts where the high-frequency current IL hardly flows because of relatively higher impedance.
As schematically illustrated in FIG. 3, a feature of the voltage resonance (type) inverter is that when the switching device Q1 is turned on and turned off, the voltage between the collector and the emitter Vce (i.e., the voltage between both ends) of the switching device Q1 is 0 V. In other words, while the voltage between the collector and the emitter Vce is 0 V, the switching device Q1 is turned on and turned off. Because of this feature, it may become possible to reduce the loss in the switching device Q1.
As schematically illustrated in FIGS. 3 and 4, there are the following four voltage resonance states (I) to (IV) generated by turning on and off the switching device Q1.    State (I): When the drive voltage VG is set to a high level so that the switching device Q1 is turned on, the commercial voltage having been transformed into DC voltage is applied between ground and one end of the exciting coil 101, the one end of the exciting coil 101 being opposite to the other end connected to the switching device Q1. As a result, the high-frequency current IL starts flowing through the exciting coil 101. Further, a desired turned-on time Ton (see FIG. 4) is set so as to obtain (apply) a desired power level. During the turned-on time Ton, the high-frequency current IL linearly increases.    State (II): After the desired turned-on time Ton has elapsed, the drive voltage VG is set to a low level so that the switching device Q1 is turned off. Then, a counter electromotive voltage is generated in the exciting coil 101, and the high-frequency current IL flows to start charging the resonance capacitor Cres. This charging process continues until the energy in the exciting coil 101 becomes zero (runs out). At that timing, the voltage Vice between the collector and the emitter of the switching device Q1 has its peak value.    State (III): Since the switching device Q1 is still turned off, the energy charged in the resonance capacitor Cres starts being discharged to the exciting coil 101. This discharge continues until the energy in the resonance capacitor Cres becomes zero (i.e., until the voltage Vce between the collector and the emitter of the switching device Q1 becomes zero). The operations in the states (II) and (III) correspond to resonance operations having the characteristics determined by the exciting coil 101 and the resonance capacitor Cres. More specifically, a turned-off time Toff is determined based on the inductance of the exciting coil 101 and the capacitance of the resonance capacitor Cres. During this state (III), the high-frequency current IL decreases in a sine waveform.    State (IV): When the discharge from the resonance capacitor Cres is finished, the diode D1 is turned on due to the counter electromotive voltage generated in the exciting coil 101. As a result, the high-frequency current IL flows from the diode D1 to the exciting coil 101. Therefore, during this state (IV), the high-frequency current IL linearly increases.
As described above, in the voltage resonance (type) inverter, the switching device Q1 is turned on while the voltage Vce between the collector and the emitter of the switching device Q1 is zero (zero voltage switching). Further, in the voltage resonance (type) inverter, generally, the frequency control is performed so as to obtain a desired temperature and a desired power level by controlling a harmonic current by controlling the length of the turned-on time Ton while the turned-off time Toff is set to be constant.
However, the inductance of the exciting coil 101 is determined based on a combination of the exciting coil 101 and the heating roller 102. More specifically, the inductance of the exciting coil 101 may vary depending on the temperature conditions of the exciting coil 101 and the heating roller 102. Because of this feature, when the inductance value of the exciting coil 101 changes by the temperature increase of the exciting coil 101 and the heating roller 102 due to the induction heating, the resonance frequency of the LC parallel resonance circuit including the exciting coil 101 and the resonance capacitor Cres varies (fluctuates). In FIG. 4, the part labeled “turned-on timing” denotes a range where the turned-on timing (i.e., end of the turned-off time Toff) varies due to the change (fluctuation) of the resonance frequency of the LC parallel resonance circuit.
Because of this feature, for example, as illustrated in FIG. 5, when the setting of the turned off timing (turned-on timing) is delayed, the switching device Q1 may be turned on or turned off while the voltage Vice between the collector and the emitter of the switching device Q1 is not zero volts. As a result, the energy charged in the resonance capacitor Cres may be discharged in, for example, a spike current to ground (GND) via the switching device Q1. Namely, the energy charged in the resonance capacitor Cres may not be converted into the energy to heat the heating roller 102 but may be lost in the switching device Q1 or may cause a temperature increase of the switching device Q1 or damage to the switching device Q1.
To overcome the problems, Japanese Patent No. 3902937 proposes a method to prevent an over-current when the switching device is turned on by calculating and setting an appropriate time period of the turned-on time and an appropriate time period of the turned-off time based on the detected value of the input voltage of the voltage resonance (type) inverter and the detected value of the temperature of the heat roller.
However, to respond to a recent strong demand for increasing the heating speed of the heating roller to reduce the heating time by the induction heating and improving the efficiency, the inductance may vary faster than ever. Therefore, when it is desired to control both the time period of the turned-on time and the time period of the turned-off time by performing a conventional calculation process and a conventional pulse width (length) setting process, the series of processes may not catch up (follow) the faster change of the inductance and be delayed. As a result, the energy loss in the switching device and the likelihood of damaging the switching device may be increased. Further, when such a fast calculation is desired to be performed, the cost of the control circuit may be increased.